1. Field of the Invention
The present invention relates to an optical fiber which can be suitably used as an optical transmission path and a dispersion compensator.
2. Related Background Art
FIG. 12 is a cross-sectional view of an optical fiber having so-called microstructures which has been known conventionally. As shown in FIG. 12, this optical fiber has a cross-sectional structure having a large number of voids 72 in a silica glass 71. A central portion in cross section having no voids 72 constitutes a core region 73 and a portion surrounding the core region 73 which has a large number of the voids 72 constitutes a cladding region 74.
The principle of light confinement of the optical fiber having such a microstructure is explained qualitatively using a concept called effective refractive indices (for example, T. A. Birks et al. optics Letters Vol. 22 p.961 (1997)). Due to the existence of the microstructure, in a strict sense, the refractive indices in the core region 73 and the cladding region 74 should have a complicate distribution. However, on the assumption that the optical guide characteristics can be approximated by replacing respective regions with uniform mediums, the refractive indices of these uniform mediums are called the effective refractive indices. The effective refractive indices neff satisfy a following equation.                                           (                                                            f                  1                                                  n                  1                  2                                            +                                                f                  2                                                  n                  2                  2                                                      )                                -            1                          ≤                  n          eff          2                ≤                                            f              1                        ⁢                          n              1              2                                +                                    f              2                        ⁢                          n              2              2                                                          (        1        )            
where n is the refractive index and f is the volume fraction. Further, a suffix 1 indicates silica glass and a suffix 2 indicates air. With respect to the volume fraction, f1+f2=1 holds. Usually, since n1 greater than n2, the both side members of the equation (1) become smaller corresponding to the increase of f2. Accordingly, the effective refractive index of the cladding region 74 having a large number of voids 72 becomes smaller than the effective refractive index of the core region 73 so that the light confinement is realized in the same manner as the usual optical fiber.
Such a model of the effective refractive indices is considered to be reasonable in a case that the optical wavelength is large compared to the scale of the microstructure. However, as the optical wavelength becomes shorter, the light is locally concentrated at portions having the high refractive index and hence, although the effective refractive indices are elevated, simultaneously, it is considered that the assumption that the structure having refractive index distribution can be replaced by the uniform mediums will lose the validity.
On the other hand, an optical fiber having a greater negative dispersion than such an optical fiber is disclosed U.S. Pat. No. 5,802,236, for example. Although this optical fiber has a similar microstructure, the optical fiber is characterized in that a cladding region is constituted by an inner cladding region and an outer cladding region and the effective refractive index of the inner cladding region is smaller than the outer cladding region.
However, although the optical fiber disclosed in the above-mentioned publication increases the negative dispersion compared to the optical fiber having the uniform cladding structure, the optical fiber suffers from a drawback that the effective core area is decreased.
The invention has been made in view of the above and it is an object to provide an optical fiber which can make the negative dispersion thereof greater than an optical fiber having a uniform cladding structure so as to increase the effective core area.
An optical fiber according to the present invention has a core region and a cladding region, surrounding the core region, comprising a main medium and sub mediums having different refractive indices from the main medium and spaced apart in the main medium. The core and cladding regions are extending along a fiber axis. The core region comprises of a central core region having a hollow portion disposed at the center of the core region and extending along the fiber axis, and an outer core region surrounding the central core region having a mean refractive index higher than the central core region and extending along the fiber axis. And the core region has a higher mean refractive index than the cladding region.
According to this configuration, the optical fiber according to the present invention has more negative waveguide dispersion than the optical fiber having a uniform cladding structure. Further, the effective core area can be increased. Accordingly, the optical fiber can attain the larger negative waveguide dispersion and at the same time can ensure the larger effective core area compared to the conventional optical fiber.
In this optical fiber, the main medium constituting the core and cladding region may be silica glass and the ratio of the optical power of the fundamental mode propagating in the hollow portion to the total power of the fundamental mode may be 1% or higher, and more preferably 10% or higher at a predetermined wavelength.
Such an optical fiber is suitable for realizing an optical transmission path of low nonlinearity and low transmission loss. In the prior art, almost all the optical power of the fundamental mode propagating in the optical fiber propagates in the inside of the main medium and hence, the nonlinearity and the transmission loss of the optical fiber become substantially equal to those of the main medium. However, by increasing the ratio of optical power of the fundamental mode propagating in the hollow portion, the nonlinearity and the transmission loss of the optical fiber take intermediate values between those of the hollow portion and the main medium. Then, by filling an inactive gas or a dry air having low nonlinearity and low transmission loss into the hollow or by keeping the hollow portion under vacuum, an optical transmission path of low nonlinearity and low transmission loss can be realized. Silica glass may be doped with Ge, F, B, P, Ti or the like so as to change the transmission characteristics of the optical fiber.
It is preferable that at least one end of the hollow portion is closed and it is more preferable to dispose hollow portions having both ends closed periodically along the fiber axis.
Due to such configuration, the occurrence of the transmission loss derived from the intrusion of contaminants into the hollow which opens at the fiber end can be prevented. Further, with the presence of portions where the hollow portions are closed, it becomes easy to cut the fiber at these portions and to realize the optical coupling between the fiber and other optical part.
The inside of the hollow may be kept under vacuum or may be preferably filled with a gas having optical gain characteristics such as H2 and NH3. By keeping the hollow portion under vacuum, the low nonlinearity and the low transmission loss can be realized. Further, by filling the gas having the optical gain characteristics into the hollow, an optical amplifying fiber can be realized. In such an optical fiber, since the ratio of the optical power propagating in the main medium is low, the nonlinear optical effect hardly occurs even when the optical power is increased to the high power.